Algal blooms occur naturally, but human development has knocked the natural nutrient cycling out of balance and made them harmful.

These harmful algal blooms (HABs) occur when water is overloaded with nutrients, especially major ones like nitrogen and phosphorus that are necessary for plant growth, but become pollution in excess. To understand where nutrient pollution in water comes from, one of the primary places to look is land use. Some of the significant sources of nutrients include wastewater treatment plant discharges, septic system leaks, fertilizer runoff from residential lawns, fertilizer runoff from farm fields and runoff or leaks from animal agriculture manure lagoons.

Depending on where HABs occur, the mix of serious contributors to nutrient pollution can change. On Long Island, New York’s Peconic River estuary, HABs are caused by excessive nitrogen pollution from faulty septic systems, wastewater treatment plant discharges and agricultural fertilizer runoff. In 2016, the severe HABs that flowed from Florida’s Lake Okeechobee – one of the United States’ largest lakes – and extended by way of two rivers to estuaries on either coast, have a host of nutrient pollutionsources like sewage septic systems and lawn runoff, as well as agricultural runoff from farm fertilizers and dairy factory farms.

To a great extent, nutrients from agricultural runoff are the big historical and present day source of pollution in the Lake Okeechobee, and it’s not an isolated case. “[T]his is not just an Okeechobee problem. It’s worldwide. Everywhere we farm, humans have always concentrated nutrients. That’s what we do,” said Paul Gray, science director of Audubon Florida’s Lake Okeechobee program.

There are many steps that can be taken to reduce nutrient pollution, from better monitoring of sources of contamination, to stricter and more tightly enforced pollution rules on wastewater treatment plants and farm fertilizer to restoring wetlands around farm fields. People at home can do things like reduce use of lawn fertilizers, buy food from producers that use sustainable methods to mitigate runoff and urge leaders to pay attention to this pressing issue. These solutions and more are available to cut nutrient pollution and fend off the scourge of Algal Doom. (Click infographic to see a larger PDF version.)